Process for the production of polyamide castings

ABSTRACT

A PROCESS FOR THE PRODUCTION OF POLYAMIDE CASTINGS, CONTAINING CAVITIES OR FITTED WITH A METAL SHAFT, BY AN ACIVATED ANIONIC POLYMERIZATION OF LACTAMS IN A MOULD, THE MOULD BEING FITTED WITH A COMPOSITE CORE OR METAL SHAFT, THE CORE OR METAL SHAFT HAVING AN OUTER INFLATABLE JACKET PREPARED BY WINDING AN ELASTIC TUBE AROUND THE INNER CORE OR METAL SHAFT WITH NEIGHBORING TURNS IN ENGAGEMENT, INSERTING THE COMPOSITE CORE OR METAL SHAFT INTO A PREHEATED STATIONARY MOULD, INFLATING THE TUBE WITH COMPRESSED GAS, FILLING THE MOULD WITH A LACTAM MELT CONTAINING AN ALKALINE CATALYST, PROMOTER, AND/OR PIGMENTS, AND/OR FILLERS, AND/OR OTHER ADDITIVES, THE LACTAM MELT THEN BEING POLYMERIZED AT A TEMPERATURE LOWER THAN THE MELTING POINT OF THE POLYLACTAM FORMED, AND AFTER COOLING, THE COMPOSITE CORE INCLUDING SAID OUTER INFLATABLE JACKET IS REMOVED FROM THE FINISHED CASTING TO GIVE A CASTING WITH A CAVITY OR IN WHICH THE METAL SHAFT TOGETHER WITH THE OUTE INFLATABLE JACKET REMAINS TIGHTLY GRIPPED IN THE CASTING.

United States Patent Office 3,658,975 PROCESS FOR THE PRODUCTION OFPOLYAMIDE CASTINGS Jan Drabek, Oto Horak, Zdenek Ditrych, and SvatoslavZahorovsky, Pardubice, Czechoslovakia, assignors to Vyzkumny ustavsyntetickych pryskyric a laku, Pardubice, Czechoslovakia N Drawing.Filed Sept. 11, 1969, Ser. No. 857,204 Claims priority, applicationCzechoslovakia, Sept. 11, 1968, 6,359/ 68, 6,366/ 68; Aug. 11, 1969,5,559/69 Int. Cl. B21d 39/00; B29c 1/12; B29d 3/00 US. Cl. 264-94 ClaimsABSTRACT OF THE DISCLOSURE A process for the production of polyamidecastings, containing cavities or fitted with a metal shaft, by anactivated anionic polymerization of lactams in a mould, the mould beingfitted with a composite core or metal shaft, the core or metal shafthaving an outer inflatable jacket prepared by winding an elastic tubearound the inner core or metal shaft with neighboring turns inengagement, inserting the composite core or metal shaft into a preheatedstationary mould, inflating the tube with compressed gas, filling themould with a lactam melt containing an alkaline catalyst, promoter,and/or pigments, and/or fillers, and/or other additives, the lactam meltthen being polymerized at a temperature lower than the melting point ofthe polylactam formed, and after cooling, the composite core includingsaid outer inflatable jacket is removed from the finished casting togive a casting with a cavity or in which the metal shaft together withthe outer inflatable jacket remains tightly gripped in the casting.

The invention relates to a new process for the production of polyamidecastings containing cavities or fitted with a metal shaft whichcomprises an activated anionic polymerization of a lactam or a mixtureof lactams in a mould. Prior to the actual polymerization, the core orthe metal shaft is provided with an outer inflatable jacket prepared bywinding an elastic tube around the inner core or metal shaft withneighboring turns in engagement. When producing castings containingcavities, the composite core is removed after polymerization and coolingof the casting. In castings fitted with a metal shaft, the metal shaftremains together with the outer inflatable jacket tightly gripped by thecasting. In both cases the high shrinkage stress ordinarily resulting incracks of the castings is eliminated.

When producing polyamide castings containing cavities by casting meltedlactam in the presence of an alkali catalyst and promoter directly onmetal cores, followed by polymerization, the considerable shrinkage ofcastings during polymerization followed by cooling from polymerizationtemperature to room temperature is a cause of many disadvantages. Thetotal volume shrinkage amounts to approximately linear shrinkage beingabout 4-5%. As the thermal dilatation of the metal core is approximatelyone order lower for the difference between the polymerizationtemperature and room temperature, e.g, steel 0.2%, the casting grips thecore so tightly that it cannot be removed. The casting may even break(crack), if the strain caused by this shrinkage is higher than thetensile strength of polyamide. For this reason it is usually necessaryto bore the desired hole in the finished casting, which means a loss ofmaterial as well as further drilling costs. Aside from this, the solidcasting must be cooled very slowly, while Patented Apr. 25, 1972castings containing holes may be cooled much more rapidly. The danger ofinternal stress is much less in castings containing holes than in solidcastings having no holes.

During the production of castings containing a metal shaft, e.g.calender bowls and gear-wheels, such high stresses are formed that thecastings nearly always crack, and such cracking cannot be prevented evenby long-time tempering. Therefore it is necessary to produce castingcontaining a metal shaft in such manner that inthe first stage a solidcasting from alkalipolyrnerized lactam is produced. A hole is thenformed in this solid casting by mechanical means, which hole may befitted with -slots'. The casting worked in this way is warmed and thenattached to the shaft. This procedure is expensive and lengthy. GermanPat. No. 1,214,865 describes the production of calender bowls coatedwith a layer of alkali polym erized lactams by rotational casting, whichprocess removes the shrinkage of the casting in the directiontowards-the metal shaft. But since the process requires a centrifugalacceleration of at least 500 g. and the axial shrinkage of the surfaceis eliminatedby the position of the rotation axis, the requirements formachinery equipment are very high. According to US. Pat. No. 3,184,828the effect of shrinkage during the production of calender bowls fromalkaline polymerized lactams is eliminated by a deformable metal jacketon the shaft of the calender bowl, while either the shaft is fitted withaxial slots and the jacket has a circular cross-section or the metaljacket having axial slots or ribs is fitted on the shaft, which has acircular cross-section. The sealing of the axial cavities thus formed onthe shaft towards the mould is not solved by this patent, although thesuccess of the process is totally dependent ,on it, because if thelactam melt poured into the mould penetrates into these cavities, thenthe effect of the deformable jacket is nullified. Aside from thisdifiiculty, the production of the deformable metal jacket is complicatedand expensive.

All the deficiencies set forth above are obviated by the presentinvention, which relates to a process for the production of polyamidecastings containing cavities or fitted with a metal shaft by the processof using an activated anionic polymerization of lactams in a mouldcontaining a composite core or a metal shaft. According to the pres entinvention'the core or metal shaft is provided with an outer inflatablejacket prepared by winding an elastic tube around the inner core ormetal shaft with neighboring turns in engagement. The tube is inflatedwith compressed gas, and said' composite core or metal shaft is insertedinto a preheated stationary mould, the mould is then filled with meltedlactam or a mixture of lactams containing an alkali catalyst, promoter,and optionally pigments and/or fillers and/or other additives. Thisreaction mixture is polymerized at a temperature lower than the meltingpoint of the polylactam formed thereby. After cooling, the compositecore including the outer inflatable jacket is removed from the castingto give a casting with a cavity or the tightly gripped metal shafttogether with the outer in- 1 During the production of castingscontaining a metal shaft,

the pressure formed in the elastic tube is progressively lowered duringthe polymerization and the cooling is carried out simultaneously withand according to the shrink-' age of the casting. The pressure in theelastic tube is" exerted by air compressed to l'20 atm.

., The stationary mould used in the process of the invention is made ofmetal and preheated to a temperature of 120-200 C., preferably 130-160C. The melt of lactam or-mixture of lactams is heated to a temperatureof 80- 180? C., preferably 120-150 C. 6-caprolactam or its mixture withw-laurolactam are the lactams most frequently and preferably used forthe purpose of the present invention. The polymerization isv effected inthe presence of 0.1-2 mole percent, preferably 0.250.4 mole percent, ofan alkaline catalyst, which is preferably an alkaline salt of a lactam,such as the sodium salt of 6-caprolactam. The polymerization isaccelerated by the addition of 0.05- 5 mole percent, preferably0.25-0.5' mole percent, of a promoter, preferably an acyl compound, suchas N-acetyl- 6-caprolactam, or an isocyanate, such as hexamethylene-1,6-diisocyanate or triphenylmethane4,4,4"-triisocyanate.

The present process for the production of castings from alkalineactivated polyamide as disclosed and claimed herein ofiers a number ofadvantages in comparison with processes known before. Castingscontaining transverse or blind holes can be produced by direct castinginto moulds containing cores. There is no danger of high internalstresses being formed andremoval of the cores from the castings is mucheasier. The influence of shrinkage can be eliminated in any requireddirection, particularly by the use ofthe inflatable hollow jackets, heldunder excess pressure as described herein.

The castings fitted with a metal shaft are produced according to thepresent invention by a single operation in contrast to the threeoperations necessary for the known and previously used procedures, i.e.production of the casting, mechanical working by boring a hole therein,followed by fitting the worked casting onto a metal shaft. Theproduction by the present invention does not require any complicatedequipment such as is necessary for rotation casting. The processaccording to the invention facilitates the production of polyamidecastings fitted with a metal shaft with a reproducible tension, by whichthe casting grips the metal shaft, depending on the tangential forces,which the metal shaft has to transmit onto the casting.

6capro1actam is the most frequently used and most desirable lactam forthe process according to the inven.

- The alkaline activated polymerization of these lactams is most oftenactivated by alkaline reacting substances (alkali metals, alkaline earthmetals or the hydrides, oxides, hydroxides, alcohols, amides, salts, andother derivatives thereof), Grignard reagents, and organometalliccompounds. The most important catalyst isnan alkaline salt of a lactam,such as the sodium salt of 6-caprolactam,

which may be added to the lactam reaction mixture either directly or maybe formed before the beginning ofthe polymerization in situ. The amountof initiators can. vary lactam or mixture of lactams being polymerized.

Examples of the preferred promoters are the acyl compounds(N-acyllactams, N,N-diacyl-bis-lactams, cyclic imides of dicarboxylicacids, and the like) and. the isocyanates (butylisocyanate,hexamethylene-1,6-diiocyanate, toluylendiisocyanates,4,4-diphenylmethanediisocyanate, triphenylmethan-4,4,4T '-triisocyanate,and the like). Other suitable and useful activators are carbodiimides,anhydrides, chlorides, hydrazides, amides, and esters of acids,lactones, imines, substituted ureas, nitrogenous, hetero? cyclic andother compounds, and various ethers and ke- 4. tones. The amount thereofrequired for promoter usage varies from about 0.05 to about 5 molepercent, preferably from 0.25 to 0.4 mole percent, based on the lactamor mixture of lactams being polymerized. The useful promoters can beused either separately or as a mixture or can be formed in situ in thecourse of the polymerization. Most frequently used promotersare theN-acyllactams, particularly N-acetyl-6- caprolactam.

To control the course of the polymerization process, it is possible andmay be desirable to add a third component into the catalyst and promotersystem, for example an alcohol, phenol or amine, which prolongs thecatalytic effect of the system and controls the value and distributionof the medium molecular weight of the polylactams.

' It is possible to add optionally reinforcing and nonreinforcingfillers and pigments (glass, asbestos, metal, cellulose and syntheticfiber materials, metal salts and oxides, metallic powders, carbon black,graphite, cement, silica, cadmium, zinc sulphide, molybdenum sulphide,various macromolecular substances, such as polyolefins,polytetrafluoroethylene, and the like). Furthermore, it is also possibleto include such known additives as stabilizers, lubricants,plasticizers, blowing agents, solvents, surface active agents, and thelike.

Substantially sealed moulds, made of material thermally stable up to 250C., are suitable for the production of the castings according to thepresent invention. Particularly convenient are aluminum and aluminumalloys, nickel and chromium. It is also possible to use ceramic orlaminated moulds and the like. Into the mould there are insertedsuitable cores, preferably made of metal, or a metal shaft, usually madeof steel.

"The composite cores or metal shafts used in accordance with the presentinvention are provided with an outer jacket made of an elasticinflatable tube prepared from an elastic material, which is capable ofwithstanding temperatures'upto about 200 C., which does not affect thepolymerization of the lactam unfavorably and which does not have anaffinity for the reaction mixture. It is possible to use thermostablemacromolecular materials, such as silicone rubber, fluoroplastics,polyimides, polysulphones, thermostable, synthetic rubbers, and thelike. A pressure of 1-20 atm., preferably 1- 5 atm., is produced in t eclascastings.

The production of polyamide castings according to the present inventionis usually carried out in the following manner:

A composite core of metal shaft, equipped with an outer inflatablejacket, made of elastic tubes, is inserted into a stationary mould. Theelastic tube is inflated by gas to a suitable pressure. The lower partof the mould, containing the composite core or the metal shaft, issubstantially and carefully sealed, its upper part is closed by a loosecover.

The mould containing the composite core or metal shaft isplaced intoappropriate heating equipment and is warmed to-the desiredtemperature. Amixture melt containinga lactam or a mixture of lactams, an alkalinecatalyst, a promoter and other additives, if and as desired, is

from about 0.1 to about 2 mole percent, preferably from 1 Poured throughfining tube inserted in the cover and 0.25 to 0.4 mole percent, theamount being based on the.

reaching to the bottom of the mould. After the mould is filled, thefilling tube is removed and the temperature in the heating equipment israised to the selected and appropriate polymerization temperature.

' When castingscontaining cavities are produced, the pressure is removedfrom the elastic tube and the composite core is extracted together withthe outer inflatable jacket.

5. When producing castings containing a metal shaft, the pressure in theelastic tube is gradually lowered during the polymerization and thecooling is effected according to the shrinking characteristics of thecasting. The Casting grips the metal shaft firmly together with theouter inflatable jacket.

The present invention facilitates the production of castings made ofpolyamides produced by anionic activated polymerization, containing oneor more cavities or metal shafts. The invention process can be used forthe production of bearings, linings, propellers, carrying rollers, cams,flanges, packings and the like. The process according to the inventionis well adapted for the production of rolls covered by a polyamide layerfor textile, paper, tanning and printing machines. In these applicationsthe outstanding properties of polyamides, such as retention of shape athigh temperatures, high abrasion resistance, non-sensitivity to passageof flashes, and the like, find extensive utilization. Other applicationsare in the production of large gears, trolley wheels for cranes,cableways and similar, i.e. in any instance or circumstance where it isnecessary to transmit forces from the shaft to the polyamide casting andat the same time utilize the outstanding properties of polyamides.

The following examples are given only for the purpose of illustratingthe present invention, wherein stands for C., and for mole percentsunless otherwise indicated.

EXAMPLE I Two cores around which elastic tubes made of a silicone rubberof 1 mm. thickness were previously wound with neighoring turns inengagement are fitted by means of screws to the mould. These tubes hadbeen previously blinded on one end by cold curable silicone rubber andthe first turn (containing the blinded end) was glued by the samematerial at the face of the cores. A similar method was used at the lastturn (at the walls of the mould) and the inlets of compressed air wereintroduced into both tubes through the openings in the wall of the mouldand sealed in these openings by cold curable silicone rubber. The mouldis loosely closed by a cover through the openings of which the fillingtube is inserted into the mould. The mould assembled in this way is putinto the heating equipment and heated to a temperature of 135 :5. Bothtubes are inflated through the inlets by compressed air in such a way,that the outer dimensions of the jacket formed by these are at least 68%greater than the corresponding dimensions of the core increased by aquadruple of the thickness of the walls of the tube. The mould is thenfilled with anhydrous 6-caprolactam, 125 :5 warm, containing 0,50% ofthe potassium salt of 6-caprolactam and 0.18 ofhexamethylene-1,6-diisocyanate. After the mould is filled, the fillingtube is removed and the temperature in the heating equipment is raisedto 190:5 during 15 minutes. Afterwards the mould is let to cool during36 hours to about 20". About 1 hour after the end of the polymerization,the excess pressure from the tubes is released. After cooling to about20, the mould containing the casting is taken out of the heatingequipment and after unscrewing the screws and releasing the inlets ofcompressed air, the casting is taken out of the mould together with bothcores and the tubes. The last step is the removing of both cores andtubes from the casting.

EXAMPLE II Into a steel mould for casting a gearwheel there is insertedthe metal shaft of this wheel. The metal shaft had been previously woundby a tube made of silicone rubber, so that the neighboring turns are inengagement. The lower part of the tube was blinded in advance by coldcurable silicone rubber, its upper part being fixed on the metal shaftby a suitable clamp. The metal shaft, provided with the tube, is fixedin the steel mould by means of a packing made of silicone rubber. Thesteel mould is closed by a loosely placed cover, through which passesthe upper part of the wound elastic tube and the filling tube. The mouldassembled in this way is warmed in suitable heating equipment to thetemperature of 135 :5 C., the wound elastic tube is filled withcompressed air to a pressure of 1.5 atm. and through the filling tube,anhydrous 6-caprolactam, heated to :5 and containing 0.40% of thepotassium salt of 6-caprolactam and 0.18% ofhexamethylene-1,6-diisocyanate, is fed into the assembled steel mould.After the mould is filled, the filling tube is removed and thetemperature of the heating equipment is raised to 190:5 C. during 15minutes. The casting is then allowed to cool to a temperature of about20 C. during 24 hours. After this period, the steel mould containing thecasting is removed from the heating equipment and its cover is taken offtogether with the packing. The gear-wheel, joined firmly with the metalshaft and the wound tube, is taken out of the steel mould. All air hasbeen expelled from the tube by shrinkage during polymerization andcooling, so that a firm joint between the metal shaft and the casting isobtained without causing such stress in the casting that it might bespontaneously damaged.

EXAMPLE III The metal shaft of a calender bowl is connected by means ofa sealing and a nut to a steel mould, open on top. The shaft is providedwith a closely adhering jacket made of foamed silicone rubber (s=0.5kg./dm. of 5 mm. thick. The mould containing the shaft is heated in asuitable heating equipment to the temperature of :5 C. Then a fillingtube reaching up into the bottom is inserted and the mould is filledwith the reaction mixture. This mixture contains 65% of anhydrouscaprolactam and 35% of anhydrous laurolactam, heated to 130:5 C. andcontaining 0.33% of the lithium salt of 6-caprolactam and 0.15% oftriphenylmethan-4,4,4"- triisocyanate. After the mould is filled, thefilling tube is removed and the temperature of the heating equipment israised during 15 minutes to 180:5 C. At the end of this heating period,the polymerization is complete and the mould containing the caSting isallowed to cool to a temperature of 20-30 C. for 48 hours. The copolymercasting, joined firmly with the metal shaft by means of the compressedlayer of foamed silicone rubber, use of which eliminates the influenceof shrinkage during polymerization and cooling, is taken out.

EXAMPLE IV The metal shaft of a calender bowl, provided with a closelyfitting hollow jacket made of silicone rubber, the walls of which are 1mm. thick and provided with an inlet tube for compressed air, is fixedin the mould by means of a gasket ring, made of silicone rubber, and anut. The hollow jacket is joined with the gasket ring by a coldvulcanizable silicone rubber adhesive. A filling tube is introduced intothe mold, the top of which is closed by a loosely placed cover. Afterthe mould has been warmed in suitable heating equipment to thetemperature of 145 :5 C., the hollow jacket is filled with air,compressed to 1.2 atm. Then the mould is filled through the filling tubewith anhydrous 6-caprolactam, heated to 135:5 C. and containing 0.35% ofNa-salt of 6-caprolactam and 0.30% of N-acetyl-6-caprolactam. After themould is filled, the filling tube is removed and the temperature of theheating equipment is raised to :5 C. during 20 minutes. After thisperiod, during which the polymerization is completed, the mouldcontaining the casting is allowed to cool to a temperature of 20 C. for36 hours. After the cooling period, the mould containing the casting istaken from the heating equipment, the cover is removed and, afterunscrewing the nut, the calender bowl is removed from the mould. Thehollow jacket is compressed by the shrinkage during polymerization andcooling so that all air has been expelled from the hollow jacket andthere is only a layer of compressed silicone rubber between the metalshaft and the casting.

EXAMPLE V Into a steel mould for casting a gear-wheel there is insertedthe metal shaft of this wheel. The metal shaft had been previously woundby a tube made of silicone'rubber. The lower part of the tube wasblinded in advance by cold curable silicone rubber, its upper part beingfixed on the metal shaft by a suitable clamp. The metal shaft, providedwith the tube, is fixed in the steel mould by means of a packing made ofsilicone rubber. The steel mould is closed by a loosely placed cover,through which passes the upper part of the tube and the fillin-g tube.The mould assembled in this way is warmed in suitable heating equipmentto the temperature of 1351- C., the tube is filled with compressed airto a pressure of 1.5 atm. and through the filling tube, anhydrous6-caprolactam, heated to 125i5 C. and containing 0.40% of the potassiumsalt of 6-caprolactam and 0.18% of hexamethylene-1,6- diisocyanate, isfed into the assembled steel mould. After the mould is filled, thefilling tube is removed and the temperature of the heating equipment israised to 190:5 C. during minutes. The casting is then allowed to coolto a temperature of about C. during 24 hours. After this period, thesteel mould containing the casting is removed from the heating equipmentand its cover is taken off together with the packing. The gearwheel,joined firmly with the metal shaft, is taken out of the steel mould. Allair has been expelled from the tube by shrinkage during polymerizationand cooling, so that a firm joint between the metal shaft and thecasting is obtained without causing such stress in the casting that itwould be spontaneously damaged.

What is claimed is:

1. In a process for the production of polyamide castings, containingcavities or fitted with a metal shaft, said polyamide being the productof activated anionic polymerization of lactams in a mould, theimprovement which comprises fitting said mould with a composite core ormetal shaft provided with an outer inflatable jacket prepared by windingan elastic tube around the inner core or metal shaft with neighboringturns in engagement, inserting said composite core or metal shaft into apreheated stationary mould, inflating said elastic tube with compressedgas, filling said mould containing said composite core or metal shaftwith a melted mixture containing a lactam or a mixture of lactams, analkaline catalyst, promoter and optionally pigments and/or fillersand/or other additives, polymerizing said mixture at a temperature lowerthan the melting point of the polylactam formed, and cooling theresulting casting.

2. A process according to claim 1, comprising after cooling, removingthe composite core including said outer inflatable jacket from saidcasting to give a casting with a cavity.

3. A process according to claim 1, comprising after cooling, allowingthe tightly-tripped metal shaft together with the outer inflatablejacket to remain in said casting.

4. A process according to claim 1, wherein said elastic tube is made ofa thermostable macromolecular material.

5. A process according to claim 1, wherein said elastic tube is made ofsilicone rubber.

6. A process according to claim 1, wherein during the production ofcastings containing cavities, a pressure is formed in said elastic tubebefore the polymerization and said pressure is maintained during thepolymerization.

7. A process according to claim 6, wherein the pressure is releasedduring cooling of the casting.

8. A process according to claim 6, wherein the pressure is releasedafter cooling of the casting.

9. A process according to claim 1 wherein during the production ofcastings fitted with a metal shaft, the pressure formed in the elastictube is progressively lowered during the polymerization and cooling incontrolled accordance with the shrinkage of the casting.

10. A process according to claim 1, wherein the pressure in the elastictube is exerted by air, compressed to 1-20 atmospheres.

References Cited UNITED STATES PATENTS 3,135,640 6/1964 Kepka et al.264-314 3,177,105 4/ 1965 Wiltshire 264-314 3,222,443 12/ 1965 Dames,Jr., et a1. 264-331 3,470,199 9/1969 Jung 264-310 3,493,647 2/1970Schuette et a1. 264-331 FOREIGN PATENTS 1,520,770 4/1968 France 264-314ROBERT F. WHITE, Primary Examiner I. R. THURLOW, Assistant Examiner US.Cl. X.R.

